1. Field of the Invention
This invention relates to the manufacture of potassium sulfate and more particularly to a method for the preparation of potassium sulfate by the reaction of potassium chloride and potassium bisulfate contained in an aqueous solution under moderate reaction conditions.
2. Description of the Prior Art
Potassium sulfate is a well known inorganic compound which has found great utility in many areas, for example, as a fertilizer and as an intermediate for the production of useful products. Consequently, the prior art has been aware of various methods for preparation of potassium sulfate for many years. In one common procedure, potassium sulfate has been prepared by the reaction of potash (KCl) and sulfuric acid. Usually, however, it is necessary to carry out such reactions under rather severe reaction conditions in order to eliminate the hydrogen chloride evolved during the reaction and shift the reaction toward production of the potassium sulfate. An inherent problem in carrying out this process is that it is a two step reaction involving an intermediate reaction in which the potash and sulfuric acid react to form potassium bisulfate (KHSO.sub.4) with the elimination of one mole of hydrogen chloride. It has been difficult heretofore to carry the reaction to completion and eliminate two moles of hydrogen chloride in conducting this reaction in view of the thermodynamics involved in the reactions being carried out. For these reasons, it has been necessary for the art to utilize rather extreme reaction conditions, e.g. temperatures of above 100.degree. C. and higher in order to obtain an economically attractive yield of potassium sulfate of reasonable purity from the reaction. These extreme reaction conditions however merely cause additional problems as the strong sulfuric acid and chlorine present at high temperatures cause severe corrosion problems. Therefore, a clear need remains in the art for methods by which potassium sulfate can be produced under moderate reaction conditions utilizing readily available starting materials.
In most of the prior art procedures, the key to a successful process resides in complete elimination of the HCl gas. A large body of art is available which discloses such reactions involving the production of K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4. A typical art reference of this type is U.S. Pat. No. 2,762,689 which involved the reaction of NaCl and H.sub.2 SO.sub.4 at tempertures of 200.degree. to 250.degree. C. Also in U.S. Pat. No. 2,275,825 Na.sub.2 SO.sub.4 and hydrochloric acid are prepared by the reaction of a slurry comprising water, NaCl, NaHSO.sub.4 and H.sub.2 SO.sub.4 with steam in a countercurrent reaction zone at temperatures of 120.degree. to 160.degree. C. With respect to prior art of this type, it should be noted that the aqueous sulfuric acid systems of Na.sub.2 SO.sub.4 and K.sub.2 SO.sub.4 are not generally considered to be equivalent in the recovery of desired products as they behave differently. Thus, K.sub.2 SO.sub.4 forms mixed acid salts on solution/crystallization whereas Na.sub.2 SO.sub.4 forms hydrates of either Na.sub.2 SO.sub.4 or NaHSO.sub.4 ; "Solubilities of Inorganic and Metal Organic Compounds", Seidell 4th Ed., Vol. 2, pps. 1126-27.
A patent of somewhat more pertinence to the present invention is U.S. Pat. No. 3,687,639 which discloses a process of precipitating K.sub.2 SO.sub.4 crystals from an aqueous solution of K.sub.2 SO.sub.4 by dissolving sufficient KCl in the solution to affect the solubility of the K.sub.2 SO.sub.4 and cause it to crystallize. However, this is strictly a physical procedure as the K.sub.2 SO.sub.4 is already formed. A similar reference with respect to sodium sulfate recovery is U.S. Pat. No. 2,125,624. However, these patents are not concerned with the particular reaction of the present invention.
To Applicants' knowledge, none of these prior methods have been entirely successful and there remains a clear need in the art for methods by which potassium sulfate can be prepared under moderate reaction conditions and in high purity and yield. The present invention is believed to provide a solution to this problem.